The invention relates to hybrid dispersions comprising nitrocellulose-polyurethanepolyurea particles (NC-PU dispersions) with accelerated drying behavior as compared with the known systems, and also to their use in coating materials.
Nitrocellulose or cellulose nitrate or cellulose ester of nitric acid is employed in a variety of fields of application on account of its diverse processing and end-application properties.
The prior art discloses solvent-borne nitrocellulose compositions as, for example, furniture varnishes, printing inks or overprint varnishes. These coating systems are notable for rapid drying.
In many cases, however, the desire is to replace solvent-borne systems by aqueous systems, owing to statutory requirements and also owing to aspects of occupational hygiene, health and environmental protection, while at the same time very largely retaining the desired, positive application properties, such as the rapid drying, for example.
Consequently there have been diverse attempts described in the prior art for providing nitrocellulose-containing coating systems on the basis of an aqueous carrier medium that do partly or wholly without organic solvents and coalescers.
U.S. Pat. No. 5,670,141, for example, first produces aqueous emulsions containing nitrocellulose, which requires costly and inconvenient emulsifying steps. Furthermore, these emulsions also contain solvents or plasticizers. Then, subsequently, the emulsions are combined with different polymers to form an aqueous system. In this way, mixtures are formed of independently produced nitrocellulose on the one hand with polymer dispersions on the other.
A different process becomes apparent, for example, in WO-A 92/01817, where nitrocellulose emulsions and different aqueous systems are applied in succession as a coating to a substrate such as leather, for example, hence always resulting in two or more operating steps.
In the prior art such as, for example, JP-A 03 294 370, JP-A 03 247 624 or JP-A 58-83 001, solvent-borne systems based on nitrocellulose and polyurethane are known. The nitrocellulose compositions described therein are used as dispersing media for magnetic powders.
JP-A 63-14735 discloses aqueous resin emulsions based on water-dispersible, urethane-modified vinyl polymers and nitrocellulose. The vinyl polymer possesses an ionic group and a polyurethane side chain, and serves as a dispersing medium for the nitrocellulose. The compositions described therein are employed as coating compositions. The resin emulsion is prepared by dissolving both the vinyl polymer and the nitrocellulose in an organic solvent, then adding water and dispersing the polymer using a neutralizing agent. The organic solvent can be removed before or after the dispersing operation. A disadvantage of these systems is their poor storage stability when the amount of solvent is reduced considerably by distillation.
EP 1 074 590 A2 describes coating materials comprising nitrocellulose. That invention, however, relates to 2K (2-component) applications, which have the disadvantages of the restricted processing life. Moreover, in those systems the use of plasticizers is mandatory.
EP 0 076 443 describes viscous, nitrocellulose-containing mixtures for producing aqueous coating emulsions In those mixtures too a plasticizer is mandatory.
EP 0 154 241 A2 describes systems comprising nitrocellulose that are UV-curing. A disadvantage of that process is that after thermal drying it is also necessary to carry out a second curing step.
JP 07 331 118 describes a water-based, quick-drying basecoat material for automotive refinish application, its formulation involving the addition of an aqueous nitrocellulose derivative. Normal nitrocellulose is hydrophobic and insoluble in water. In order to make the nitrocellulose water-soluble it must be hydrophilicized. This hydrophilicity is then retained subsequently in the completed coating film. This is also the reason why the hydrophilicized nitrocellulose has not become established in the market.
Described for the first time in German Patent Application No. 10 2006 012354, unpublished at the priority date of the present specification, are NC-PU dispersions with which success is achieved in incorporating inherently hydrophobic nitrocelluloses stably into aqueous dispersions and hence combining the profiles of properties of nitrocellulose coating materials and polyurethane coating materials. Deserving of improvement, however, is the comparatively slow drying behavior of films and coatings obtainable from those dispersions.
Building on DE 10 2006 012354 it has now surprisingly been found that a blend of such hybrid NC-PU dispersions with aqueous, non-nitrocellulose-containing polyurethane-polyurea dispersions (PU dispersions) leads to significantly more rapid drying as compared with NC-PU dispersions according to DE 10 2006 012354 that have the same solids content and nitrocellulose fraction.